In recent years, multilayer sheets that include a layer containing air thereinside have been used for various purposes. Examples thereof include a backlight unit of a liquid crystal display.
A backlighting system is generally employed in liquid crystal displays, and light is irradiated from a back side of a liquid crystal display. The backlighting is mainly categorized into an edge-lit type (also referred to as “side-lit type”) and a direct-backlit type. The edge-lit type backlight unit includes a light guide sheet and a light source as main components, and a multilayer sheet as described above is used as the light guide sheet.
The light guide sheet is configured to allow transmission of light therethrough. In the light guide sheet, one main surface thereof opposing to a liquid crystal unit is used as a light-emitting surface, and one lateral surface substantially perpendicular to the light-emitting surface is used as a light-incident surface. The light source is disposed so as to face the light-incident surface. Light rays emitted from the light source travel through the light guide sheet with reflections, and light rays having a relatively high NA (Numerical Aperture) with respect to the light-emitting surface are emitted from the light-emitting surface.
As a light guide sheet that includes a layer containing air thereinside, for example, there has been proposed a sheet that is configured to include a first optical layer; a second optical layer; a low-refractive index layer disposed between the first optical layer and the second optical layer, the low-refractive index layer including plural hollow particles and having a lower refractive index than those of the first optical layer and the second optical layer; and an intermediate layer disposed between the first optical layer and the low-refractive index layer (see, for example, International Publication (WO) No. 2012/105597).
In the sheet disclosed in WO 2012/105597, when light falls upon the first optical layer along a plane direction of the sheet, the light that has entered thereinto travels mainly through the first optical layer. In the low-refractive index layer, since the particles themselves have cavities and gaps are formed between the particles, the light that travels through the first optical layer is reflected at the boundary of the first optical layer and the low-refractive index layer, which results in the reduced entry of the light into the low-refractive index layer. Accordingly, since the sheet disclosed in WO 2012/105597 allows appropriate transmission of light, favorable luminance can be obtained even at a place that is distant from the light-incident surface.
In addition, in the sheet disclosed in WO 2012/10559, the intermediate layer is disposed between the first optical layer and the low-refractive index layer, so that favorable interlayer adhesion properties can be obtained.